The present invention relates to a method for generating a pixel synchronizing signal which is used for sampling or recording high density pixels in a color scanner for printing plate making etc., for example, such as picture input scanning apparatus, reproduction record scanning apparatus, and further relates to an apparatus for practicing the method.
In a recent scanning type picture reproduction recording apparatus, for example, such as a rotating drum type color scanner, in order to carry out digital processing, at the original picture scanning side a picture element (pixel) synchronizing signal which is used for sampling is necessary for converting analog picture signals into digital signals, and at the recording side another pixel synchronizing signal for reading out picture signals to record the pixels with desired recording densities is required. These pixel synchronizing signals are generated so as to be synchronized in phase with output pulse signals of a rotary encoder which is provided to generate a plurality of pulse trains per one rotation of the drum.
The output pulse of the rotary encoder of high resolving power corresponding to a sampling pitch of a pixel can not be obtained immediately by basing on mechanical structure of the rotary encoder. Accordingly, in order to obtain a pulse of high resolving power corresponding to size of a desired pixel, frequency of the output pulse signal of the rotary encoder is converted and multiplied by a phase synchronizing loop circuit (hereinafter refer to PLL).
The output pulse signal of the rotary encoder is input to the PLL circuit, and a higher frequency signal which is phase synchronized with the input pulse signal can be obtained as an output of the PLL circuit, so that variation in frequency of the input signal promptly influences on frequency variation of the output signal, by which reading out picture image data or recording the pixels synchronizes with speed of the rotary drum, even at a case when variation of speed in the main scanning direction occurs. As described the above, there is an advantage that synchronization can be carried out without any inconvenience. However, on the other hand, as shown in a conventional pixel synchronizing signal generating circuit shown in FIG. 3, a frequency divider 51 disposed in a feeding back loop a PLL circuit 50 is a digital counter, so that a counting value of a frequency counter provided for dividing frequency to obtain a frequency dividing ratio (1/A) of the frequency divider 51 must be an integer. Accordingly, an output frequency (f.sub.o) of the PLL circuit 50 cannot be converted only in a case in which the output frequency (f.sub.o) of the PLL circuit 50 must be an integral multiple of an input frequency (f.sub.i), that is, an integral value obtained by multiplied by the integer (A). The relation can be represented as the following formula, f.sub.o =A.multidot.f.sub.i. Therefore, it is impossible to adjust the output frequency (f.sub.o) extremely minute by basing on the value of the integer (A).
In addition, a repeating frequency of an output pulse signal of a rotary encoder 52, that is, since the input frequency (f.sub.i) of the PLL circuit 50 is previously determined according to the number of output pulses per one rotation of a rotating drum 53, there is no room for selecting any other frequencies but for the output frequency (f.sub.o).
According to the above described reasons, a pixel synchronizing pulse generating circuit is adapted to obtain a desired approximate frequency value in which the counting value (A) of the frequency divider 51 is made larger, the output frequency (f.sub.o) is made sufficiently higher than the desired frequency and then the frequency is divided (demultiplied) by another frequency divider 54 to lower the frequency, and with a ratio between a frequency dividing counting value (B) of the frequency divider 54 and the frequency dividing counting value (A) of the frequency divider 51 of a feeding back loop in the PLL circuit 50, that is, according to the ratio A/B.
However, the both counting values (A) and (B) must be integer or integers, and if the number of figures of the counting value of the frequency divider 51 of the feeding back loop is high, the output frequency becomes too higher and it lowers loop gain. For that reason capture range and lock range become narrower which results in being unstable in operation of the PLL circuit 50, because of the above described reason, the range of the value (A) is limited to, in the case of the input frequency (f.sub.i) being the extent of 10 KHz, a number of three figures or four figures (10 MHz-100 MHz).
Further, the reference number 53 shown in FIG. 3 designates a rotary drum at the recording side in a color scanner, the reference number 55 is a recording head which is fed to the subscanning direction by a feeding motor 56 and a feeding screw 57, the reference number 58 designates a phase comparator, the reference number 59 designates a lower pass filter, 60 designates a voltage controlling oscillator. The reference 61 is a picture image memory part in which desired digital picture image signals are stored, and the reference 62 is a picture image processing part which processes timing for feeding the picture image signals which come from the picture image memory part 61 sequently to the recording head 55.
The picture images stored in the picture image memory 61 include picture patterns scanned at the original picture scanning side and appropriate picture image generating means, for example, a mask picture image formed from such as digitizer etc. and character and picture image etc. formed by a character generating means etc., and there are some cases in which each of the picture images is synthesized appropriately prior to being stored in the picture memory part 61.
When the above described picture image synthesizing processing is carried out, infundamental shape and size of each of pixels is desired to be a square which is exact in size. For that reason the sampling pitch in scanning the original picture must be coincided with a feeding pitch in the subscanning direction.
On the other hand, the picture images read out of the picture image memory part 61 are converted in magnification as desired value, when the picture images are to be recorded, and by this magnification conversion, the pixel size corresponding to the sampling pitch at the original picture scanning time is recording with relatively magnified or reduced scale. Accordingly, it is desired that magnification ratio and/or reduction ratio can be adjusted with possible minute step. For example, assuming that sampling frequency (f') at the recording side is constant, and the sampling frequency (f') at the original picture side is variable basing on a formula f=mf' (here, m is a reproducing magnification), according to desired magnification ratio, in the apparatus of a type in which magnification conversion is carried out, if conversion step in magnification is required to stepwisely vary by every 0.001%, it is necessary for the sampling frequency (f) to vary frequency conversion step as the same ratio.
However, as described the above, the sampling pitch in the original picture scanning and the sampling pitch in the recording scanning are determined to the result of the following formula represents by the set up values (A) (B) of the frequency dividers 51 and 54. EQU f.sub.o '=(A/B).multidot.f.sub.i
Accordingly, regarding a case in which a pixel of magnification ratio 1 is recorded, descriptions will be given hereinafter in a case in which the original picture being scanned an exactly sized pixel is sampled, and a case of the recording scanning in which unevenness of sizes of diameters in the recording cylinders is found and difference in thickness of the photosensitive material(s) is also found.
Assuming that density of scanning lines is 80 lines/mm (about 2000 lines per inch), then feeding pitch in the subscanning direction is 12.5 .mu.m, and this pitch becomes length of one side of the square shaped pixel in the subscanning direction. When the number of revolution of the rotary drum 53 is assumed to be 600 rpm and effective diameter of the rotary drum (including thickness of the original picture and/or that of the recording film) is 248mm, scanning time (t) per one side (12.5 .mu.m) of the pixel in the main scanning direction can be obtained by the following formula, that is, ##EQU1## Then, an exact pixel synchronizing signal (f.sub.a) required for sampling or recording of the square shaped pixel becomes as follows; EQU l /t=623.2919825 KHz.
On the other hand, if the rotary encoder 52 outputs pulses of 1000 per one revolution number, the input frequency (f.sub.i) of the PLL circuit 50 becomes 10 KHz, and the input-output frequency of this circuit has the following, that is, f.sub.o '=f.sub.i .times.A/B and relation of f.sub.o '=f.sub.a is to be wished, so that the following relation can be found; EQU A/B=f.sub.o /f.sub.i =623.291825/10KHz
From this relation EQU f.sub.o '=f.sub.i .times.A/B
From the above described relation, if the optimum values of (A) and (B) are selected, A=1558, B=25 are used as the best approximate values. However, from the above described best approximate values of A and B which can be concretely obtained, an actual size of the pixel is obtained as follows, that is, length of the one side of the square in the main scanning direction is 12.50184496 .mu.m. When 8000 pixels of this size are arranged in the main direction, the whole length of the pixels becomes larger by 14.759 .mu.m, that is, length fo a little longer than one pixel side is increased in comparison with the whole length (100 mm) of 8000 pixels of square shaped having side length of exact 12.5 .mu.m.
While, in recording the picture image, when magnification ratio is varied by magnifying or reducing the picture images, even either +1 or -1 and unit integer value are added to or subtracted from value of the A of higher figures to obtain the minimum magnification ratio or reduction ratio, frequency transfer per one step is too large, so that magnification variation coefficient of minute step cannot be expected by the conventional circuit shown in FIG. 3.